1. Field of the Invention
The present invention relates to a method for preparing an electrolyte for a vanadium redox flow battery using vanadium oxide, and more particularly, to a method for preparing an electrolyte, in which, when a vanadium electrolyte capable of being supplied to both a positive electrode and a negative electrode of the vanadium redox flow battery is prepared, vanadium in the +3 oxidation state (V3+) and vanadium in the +4 oxidation state (V4+) are mixed in the same proportions to prepare a vanadium electrolyte having a median oxidation number therebetween, thereby maximizing battery efficiency based on the supply capacitance; and the voltage difference between the produced electrolyte and a reference electrolyte having a desired oxidation number is confirmed in real time, thereby rapidly controlling the mixture ratio of the vanadium in the +3 oxidation state (V3+) and vanadium in the +4 oxidation state (V4+) and thus simplifying an oxidation number checking procedure and shortening the preparing time, so that an electrolyte having a desired oxidation number can be rapidly prepared and the price competitiveness of products can be improved.
2. Description of the Prior Art
A redox flow battery is one of the core products, which are closely associated with energy revolution, renewable energy, greenhouse gas reduction, rechargeable batteries, and smart grids, and have currently attracted a greatest interest over the world.
Most of current energy is obtained from fossil fuel, but the use of this fossil fuel has a serious negative effect on the environment, such as air pollution, global warming, and acid rain. Moreover, this fossil fuel has low energy efficiency.
In order to solve the problems due to the use of the fossil fuel, the interest in renewable energy is rapidly increasing and researches on the renewable energy have been actively conducted in Korea as well as around the world, in recent years.
It is said that markets of the renewable energy have entered a mature stage at home and abroad. However, due to the nature of renewable energy, the generation amount of renewable energy largely varies depending on changes in environmental factors such as time and weather. Thereby, Energy Storage Systems (ESSs) storing the generated renewable energy need to be very urgently distributed. Redox flow batteries have gained attention as a large-capacity energy storage system.
An electrolyte of the redox flow battery employs a material having a redox couple. The electrolyte of the initial redox battery was used by applying redox couples of different materials to a positive electrode and a negative electrode, respectively. However, there may be a cross contamination phenomenon in which materials ionized during charging and discharging cross each other through a separator, causing capacitance reduction.
Therefore, it was supposed that vanadium could be used for both the positive electrode and the negative electrode in order to minimize the ion cross contamination, and thereby the application of vanadium to the electrolyte leads to a high electrical voltage, and a long use lifespan and thus long-term use.
That is, a vanadium redox flow battery (VRFB) incurs little consumption of materials since, during charging and discharging, the oxidation number of vanadium varies while a vanadium active material circulates a positive electrode and a negative electrode. In addition, the VRFB has been rated as the best secondary battery energy storage system (ESS) since it has the longest lifespan among secondary batteries and is advantageous for high capacity; the electrolyte can be separated and reused through an electrochemical reaction of charging and discharging even when a positive electrolyte and a negative electrolyte are mixed passing through a membrane, thereby lowering the maintenance cost.
As for the VRFB, a vanadium electrolyte used in the positive electrode and the negative electrode needs a vanadium compound having an oxidation number of +4 for the positive electrode and +3 for the negative electrode based on a complete discharge state, and needs a vanadium compound having an oxidation number of +5 for the positive electrode and +2 for the negative electrode based on a complete charge state. Therefore, the charging and discharging reaction occurs while the oxidation number of the vanadium active material varies, and it is preferable to prepare a positive electrode and a negative electrolyte to have accurate oxidation numbers in order to allow an accurate electrochemical reaction to occur.
In the case of the existing vanadium electrolyte, it is general to dissolve vanadium oxide such as V2O5, VOSO4, or V2(SO4)3 in acid such as sulfuric acid, hydrochloric acid, phosphoric acid, or nitric acid, and then allow the vanadium electrolyte to have a predetermined oxidation number by using a reducing agent. In order to prepare a vanadium electrolyte having an accurate oxidation number, a method of using a stack is used.
For example, when a vanadium electrolyte having an oxidation number of +4 is supplied to a positive electrode and a negative electrode of the stack and charging is conducted to 100% state of charge (SOC), a positive electrolyte has an oxidation number of +5 and a negative electrolyte has an oxidation number of +3 through an electrochemical reaction. Here, in order to allow charging and discharging to occur, the positive electrolyte having an oxidation number of +5 is discharged from the stack and reduced to a vanadium electrolyte having an oxidation number of +4 by using a reducing agent, so that a positive electrolyte in the +4 oxidation state and a negative electrolyte in the +3 oxidation state based on a complete discharge state can be obtained. The preparing method using the stack is advantageous in preparing an electrolyte having an accurate oxidation number, but the stack material may be damaged as the charging and discharging reaction proceeds, and the preparing time may be increased, causing a rise in preparation cost.
Further, a method of preparing a positive electrolyte and a negative electrolyte by using a reducing agent does not use the stack, resulting in a decrease in preparation cost, but since it is difficult to obtain an electrolyte having an accurate oxidation number through only a chemical reaction, the performance of the electrolyte may be deteriorated. Further, the accuracy of oxidation number is improved by applying an oxidation number checking method, but the oxidation number checking method is considerably cumbersome since the oxidation number is determined by taking the sample and then feeding a reagent, which is color-changeable at a specific oxidation number, thereto to measure the color change; the time for preparing, such as sampling and checking of the color change reaction, is increased; and a preparing process for purification needs to be added to reuse the checked sample.
Japanese Patent Laid-Open Publication no. 2004-71165 (published: 2004 Mar. 4, hereinafter, ‘publication patent’) discloses a method for preparing an electrolyte for a vanadium redox battery. Here, a vanadium-sulfuric acid solution in which vanadium having an oxidation number of +4 or higher is dissolved in sulfuric acid is used as a negative electrolyte and an aqueous solution containing sulfuric acid or a vanadium-sulfuric acid solution is used as a positive electrolyte. Then, the negative electrolyte is reduced by an electrolytic reduction apparatus until the negative electrolyte has an oxidation-reduction potential of 100˜400 mV, so that vanadium having an oxidation number of +3.5 is obtained. The foregoing method of the publication patent has an advantage of preparing a vanadium electrolyte having an oxidation number of +3.5, commonly usable as the negative electrolyte and the positive electrolyte. However, since the negative electrode and the positive electrode are respectively put in the positive electrolyte and the negative electrolyte to obtain an electrolyte having an oxidation number of +3.5 by a reducing procedure through electrolysis, the stack material may be damaged during the electrolysis. Moreover, although the preparing time may be partially decreased, the preparation procedure still takes a long time since the electrolytic reduction procedure is conducted to a desired oxidation number.